1. Field of the Invention
The present invention relates to an organic light emitting device constituted by an anode, an organic compound film capable of emitting light under the action of an electric field, and a cathode. In particular, the present invention relates to an organic light emitting device using a light emitting material which emits light in a triplet exited state.
2. Description of the Related Art
An organic light emitting device is a device designed by utilizing a phenomenon in which electrons and holes are caused to flow into an organic compound film through two electrodes by application of a voltage to cause emission of light from molecules in an excited state (excited molecules) formed by recombination of the electrons and holes.
Emission of light from an organic compound is a conversion into light of energy released when excited molecules are formed and then deactivated into the ground state. Deactivation processes causing such emission of light are broadly divided into two kinds: a process in which deactivation proceeds via a state in which excited molecules are singlet excited molecules (in which fluorescence is caused), and a process in which excited molecules are triplet excited molecules. Deactivation processes via the triplet excited molecule state include an emission process in which phosphorescence is caused and a triplet-triplet extinction process. However, there are basically only a small number of organic materials capable of changing in accordance with the phosphorescent deactivation process at room temperature. (In ordinary cases, thermal deactivation different from deactivation with emission of light occurs.) The majority of organic compounds used in organic light emitting devices are materials which emit light by fluorescence via the singlet excited molecule state, and many organic light emitting devices use fluorescence.
Organic light emitting devices using such organic compounds capable of emitting light by fluorescence are based on the two-layer structure which was reported by C. W. Tang et al. in 1987 (Reference 1: C. W. Tang and S. A. Vanslyke, “Organic electroluminescent diodes”, Applied Physics Letters, Vol. 51, No.12, 913-915 (1987)), and in which an organic compound film formed of layers of two or more organic compounds and having a total thickness of about 100 nm is interposed between electrodes. Adachi et al. thereafter proposed a three-layer structure in 1988 (Reference 2: Chihaya ADACHI, Shozuo TOKITO, Tetsuo TSUTSUI and Shogo SAITO, “Electroluminescence in Organic Films with Three-Layered Structure”, Japanese Journal of Applied Physics, Vol. 27, No.2, L269-L271 (1988)). Multilayer device structures based on applications of these layered structures are being presently used.
Devices in such multilayer structures are characterized by “layer function separation”, which refers to the method of separately assigning functions to layers, instead of making one organic compound have various functions. For example, a device of two-layer structure uses a hole transporting layer having the function of transporting positive holes, and a light emitting and electron transporting layer having the function of transporting electrons and the function of emitting light. Also, a device of three-layer structure uses a hole transporting layer having only the function of transporting positive holes, an electron transporting layer having only the function of transporting electrons, and a light-emitting layer which is capable of emitting light, and which is formed between the two transporting layers. Such a layer function separation method has the advantage of increasing the degree of molecular design freedom of organic compounds used in an organic light emitting device.
For example, a number of characteristics, such as improved facility with which either of electrons and holes are injected, the function of transporting both the carriers, and high fluorescent quantum yield, are required of a device of single-layer structure. In contrast, in the case of a device of two-layer structure or the like using an electron transporting and light emitting layer, an organic compound to which positive holes can be easily injected may be used as a material for a hole transporting layer, and an organic compound to which electrons can be easily injected and which have high fluorescent quantum yield may be used as a material for an electron transporting layer. Thus, requirements of one layer are reduced and the facility with which the material is selected is improved.
In the case of a device of three-layer structure, a “light emitting layer” is further provided to enable separation between the electron transporting function and the light emitting function. Moreover, a material in which a fluorescent pigment (guest) of high quantum yield such as a laser pigment is dispersed in a solid medium (host) material can be used for the light emitting layer to improve the fluorescent quantum yield of the light emitting layer. Thus, not only the effect of largely improving the quantum yield of the organic light emitting device but also the effect of freely controlling the emission wavelength through the selection of fluorescent pigments to be used can be obtained (Reference 3: C. W. Tang, S. A. Vanslyke and C. H. Chen, “Electroluminescence of doped organic thin films”, Journal of Applied Physics, Vol. 65, 3610-3616 (1989)). A device in which such a pigment (guest) is dispersed in a host material is called a doped device.
Another advantage of the multilayer structure is a “carrier confinement effect”. For example, in the case of the two-layer structure described in Reference 1, positive holes are injected from the anode into the hole transporting layer, electrons are injected from the cathode into the electron transporting layer, and the holes and electrons move toward the interface between the hole transporting layer and the electron transporting layer. Thereafter, while holes are injected into the electron transporting layer because of a small ionization potential difference between the hole transporting layer and the electron transporting layer, electrons are blocked by the hole transporting layer to be confined in the electron transporting layer without being injected into the hole transporting layer because the electrical affinity of the hole transporting layer is low and because the difference between the electrical affinities of the hole transporting layer and the electron transporting layer is excessively large. Consequently, both the density of holes and the density of electrons in the electron transporting layer are increased to achieve efficient carrier recombination.
As an example of a material that is effective in exhibiting the carrier confinement effect, there is a material having an extremely large ionization potential. It is difficult to inject holes into the material having a large ionization potential, so that such a material is widely used as a material capable of blocking holes (hole blocking material). For example, in the case where the hole transporting layer composed of an aromatic diamine compound and the electron transporting layer composed of tris(8-quinolinolato)-aluminum (hereinafter referred to as “Alq”) are laminated as reported in Reference 1, if a voltage is applied to the device, Alq in the electron transporting layer emits light. However, by inserting the hole blocking material between the two layers of the device, holes are confined in the hole transporting layer, so that light can be emitted from the hole transporting layer side as well.
As described above, layers having various functions (hole transporting layer, hole blocking layer, electron transporting layer, electron injection layer, etc.) are provided to improve the efficiency and to enable control of the color of emitted light, etc. Thus, multilayer structures have been established as the basic structure for current organic light emitting devices.
Under the above-described circumstances, in 1998, S. R. Forrest et al. made public a doped device in which a triplet light emitting material capable of emission of light (phosphorescence) from a triplet excited state at a room temperature (a metal complex having platinum as a central metal in the example described in the reference) is used as a guest (hereinafter referred to as “triplet light emitting device) (Reference 4: M. A. Baldo, D. F. O'Brien, Y You, A. Shoustilkov, S. Silbley, M. A. Thomoson and S. R. Forrest, “Highly efficient phosphorescent emission from organic electroluminescent devices”, Nature, Vol. 395, 151-154 (1998)). For distinction between this triplet light emitting device and devices using emission of light from a singlet excited state, the latter device will be referred to as “singlet light emitting device”.
As mentioned above, excited molecules produced by recombination of holes and electrons injected into an organic compound include singlet excited molecules and triplet excited molecules. In such a case, singlet excited molecules and triplet excited molecules are produced in proportions of 1:3 due to the difference between the multiplicities of spin. Basically, in the existing materials, triplet excited molecules are thermally deactivated at room temperature. Therefore only singlet excited molecules have been used for emission of light, only a quarter of produced excited molecules are used for emission of light. If triplet excited molecules can be used for emission of light, a light emission efficiency three to four times higher than that presently achieved can be obtained.
In Reference 4, the above-described multilayer structure is used. That is, the device is such structured that: 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (hereinafter referred to as “α-NPD”) that is an aromatic amine-based compound, is used as the hole transporting layer; Alq with 6% of 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin-platinum (hereinafter referred to as “PtOEP”) dispersed therein is used as the light emitting layer; and Alq is used as the electron transporting layer. As to the external quantum efficiency, the maximum value is 4% and a value of 1.3% is obtained at 100 cd/m2.
Thereafter, the device structure utilizing the hole blocking layer is used. That is, the device is such structured that: α-NPD is used as the hole transporting layer; 4,4′-N,N′-dicarbazole-biphenyl (hereinafter referred to as “CBP”) with 6% of PtOEP dispersed therein is used as the light emitting layer; 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (hereinafter referred to as “BCP”) is used as the hole blocking layer; and Alq is used as the electron transporting layer. As to the external quantum efficiency, a value of 2.2% is obtained at 100 cd/m2 and the maximum value is 5.6%, so that the light emission efficiency of the device is improved (Reference 5: D. F. O'Brien, M. A. Baldo, M. E. Thompson and S. R. Forrest, “Improved energy transfer in electrophosphorescent devices”, Applied Physics Letters, Vol. 74, No. 3, 442-444 (1999)).
Further, a triplet light emitting device is reported which uses tris(2-phenylpyridine)iridium (hereinafter referred to as “Ir(ppy)3”) as the triplet light emitting material (Reference 6: M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson and S. R. Forrest, “Very high-efficiency green organic light-emitting devices based on electrophosphorescence”, Applied Physics Letters, Vol. 75, No. 1, 4-6 (1999)). Thereafter, it is reported that with the same device structure as in Reference 6, the film thicknesses of the organic compound films are optimized, whereby a highly efficient organic light emitting device is obtained whose external quantum efficiency is 14.9% at 100 cd/m2 (Reference 7: Teruichi Watanabe, Kenji Nakamura, Shin Kawami, Yoshinori Fukuda, Taishi Tsuji, Takeo Wakimoto, Satoshi Miyaguchi, Masayuki Yahiro, Moon-Jae Yang, Tetsuo Tsutsui, “Optimization of emitting efficiency in organic LED cells using Ir complex”, Synthetic Metals, Vol. 122, 203-207 (2001)). Thus, in actuality, it becomes possible to produce the devices with the light emission efficiency almost three times that in the conventional singlet light emitting device.
Searches are presently being made for triplet light emitting materials using iridium or platinum as a central metal, triplet light emitting devices having markedly high efficiency in comparison with singlet light emitting devices are now attracting attention, and studies about such devices are being energetically made.
Although triplet light emitting devices have light emission efficiency much higher than that of singlet light emitting devices, they are incomparably shorter in life than singlet light emitting materials and lack stability. Also, a multilayer structure adopted to increase the efficiency of a triplet light emitting device must be formed so as to have at least four layers. Therefore triplet light emitting devices simply have the drawback of requiring much time and labor for fabrication.
With respect to the life of devices, a report has been made which says that the half-life of a device having a multilayer structure formed of a hole transporting layer using α-NPD, a light emitting layer using CBP as a host material and Ir(ppy)3 as a guest (dopant) material, a hole blocking layer using BCP, and an electron transporting layer using Alq is only 170 hours under a condition of an initial luminance of 500 cd/M2 (Reference 8: Tetsuo TSUTSUI, Moon-Jae YANG, Masayuki YAHIRO, Kenji NAKAMURA, Teruichi WATANABE, Taishi TSUJI, Yoshinori FUKUDA, Takeo WAKIMOTO and Satoshi MIYAGUCHI, “High Quantum Efficiency Inorganic Light-Emitting Devices with Iridium-Complex as a Triplet Emissive Center”, Japanese Journal of Applied Physics, Vol. 38, No.12B, L1502-L1504 (1999)). By considering this life, it must be said that no solution of the life problem is close at hand.
In Reference 8, low stability of BCP used as a hole blocking material is mentioned as a cause of the limitation of life. Triplet light emitting devices use as a basic structure the device structure described in Reference 5, and use the hole blocking layer as an indispensable element. FIG. 12 show the structure of a conventional triplet light emitting device. In the device structure shown in FIG. 12, an anode 1102 is formed on a substrate 1101, a multilayer organic compound film formed of a hole transporting layer 1103, a light emitting layer 1104, a hole blocking layer 1105, and an electron transporting layer 1106 is formed on the anode 1102, and a cathode 1107 is formed on the multilayer film. While efficient carrier recombination can be achieved by the carrier confinement effect of the hole blocking layer, the life of the device is limited because the hole blocking material ordinarily used is considerably low in stability. Also, CBP used as a host material is also low in stability and is also considered to be a cause of the limitation of the life.
A device of three-layer structure using no hole blocking layer has been fabricated (Reference 9: Chihaya ADACHI, Marc A. Baldo, Stephen R. Forrest and Mark E. Thompson, “High-efficiency organic electrophosphorescent devices with tris(2-phinylpyridine)iridium doped into electron-transporting materials”, Applied Physics Letters, Vol. 77, No.6, 904-906 (2000)). This device is characterized by using electron transporting materials as a host material instead of CBP which is the to have such characteristics as to transport both the carriers. However, the electron transporting materials used as a host material are BCP which is used as a hole blocking material, 1,3-bis(N,N-t-butyl-phenyl)-1,3,4-oxazole (hereinafter referred to as “OXD7”), and 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole (hereinafter referred to as “TAZ”). Although no hole blocking layer is used, the materials ordinarily used as a hole blocking material are used in the device. BCP is, of course, lower in stability than any other material, so that the stability of the device is low, while the efficiency is high.
A simple two-layer device structure using no hole blocking material has also been reported (Reference 10: Chihaya ADACHI, Raymond KWONG, Stephen R. Forrest, “Efficient electrophosphorescence using a doped ambipolar conductive molecular organic thin film”, Organic Electronics, Vol. 2, 37-43 (2001)). In this device, however, CBP is used as a host material, so that the stability is low, while the light emission efficiency is high.
As described above, while triplet light emitting devices having high light emission efficiency have been reported, no triplet light emitting device improved both in efficiency and in stability has been reported. Difficulty in obtaining such an improved device is due to the instability of host materials and hole blocking materials used.